U.S. patent number 6,644,023 [Application Number 10/088,477] was granted by the patent office on 2003-11-11 for exhaust emission control device of internal combustion engine.
This patent grant is currently assigned to Toyota Jidosha Kabushiki Kaisha. Invention is credited to Takamitsu Asanuma, Shinya Hirota, Kazuhiro Itoh, Koichi Kimura, Koichiro Nakatani, Toshiaki Tanaka.
United States Patent |
6,644,023 |
Hirota , et al. |
November 11, 2003 |
Exhaust emission control device of internal combustion engine
Abstract
The present device for purifying the exhaust gas of an internal
combustion engine comprises a particulate filter 70 on which the
trapped particulates are oxidized and reversing means 74 for
reversing the exhaust gas upstream side and the exhaust gas
downstream side of the particulate filter. The particulate filter
has a trapping wall for trapping the particulates. The reversing
means reverses the exhaust gas upstream side and the exhaust gas
downstream side of the particulate filter so that the first
trapping surface and the second trapping surface are used
alternately to trap the particulates. The particulate filter has a
first opening portion and a second opening portion through which
the exhaust gas flows in and out from the particulate filter and is
arranged in the exhaust pipe 71 upstream of the muffler. At least
part of the circumferential portion of the particulate filter
between the first opening portion and the second opening portion is
in contact with the flow of the exhaust gas in the exhaust
pipe.
Inventors: |
Hirota; Shinya (Susono,
JP), Tanaka; Toshiaki (Numazu, JP), Itoh;
Kazuhiro (Mishima, JP), Asanuma; Takamitsu
(Susono, JP), Nakatani; Koichiro (Susono,
JP), Kimura; Koichi (Susono, JP) |
Assignee: |
Toyota Jidosha Kabushiki Kaisha
(Toyota, JP)
|
Family
ID: |
18716932 |
Appl.
No.: |
10/088,477 |
Filed: |
March 20, 2002 |
PCT
Filed: |
July 23, 2001 |
PCT No.: |
PCT/JP01/06349 |
PCT
Pub. No.: |
WO02/08580 |
PCT
Pub. Date: |
January 31, 2002 |
Foreign Application Priority Data
|
|
|
|
|
Jul 24, 2000 [JP] |
|
|
2000-222725 |
|
Current U.S.
Class: |
60/297; 60/287;
60/296; 60/311 |
Current CPC
Class: |
B01D
46/0063 (20130101); F01N 3/0821 (20130101); F01N
3/021 (20130101); F01N 3/0842 (20130101); F01N
3/035 (20130101); F01N 3/0233 (20130101); F01N
3/0222 (20130101); B01D 46/2418 (20130101); B01D
46/0067 (20130101); F01N 13/017 (20140601); F01N
3/0211 (20130101); B01D 46/002 (20130101); F01N
2510/065 (20130101); F02D 41/029 (20130101); B01D
46/2486 (20210801) |
Current International
Class: |
B01D
46/24 (20060101); F01N 3/021 (20060101); F01N
3/035 (20060101); F01N 3/022 (20060101); F01N
3/08 (20060101); F01N 7/00 (20060101); F02D
41/02 (20060101); F01N 7/04 (20060101); F01N
3/023 (20060101); F01N 003/00 () |
Field of
Search: |
;60/311,301,297,287,296
;55/DIG.30,483,484 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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|
57-65812 |
|
Apr 1982 |
|
JP |
|
61-164014 |
|
Jul 1986 |
|
JP |
|
A 62-210212 |
|
Sep 1987 |
|
JP |
|
A 5-179928 |
|
Jul 1993 |
|
JP |
|
A 5-256121 |
|
Oct 1993 |
|
JP |
|
A 9-125931 |
|
May 1997 |
|
JP |
|
Primary Examiner: Denion; Thomas
Assistant Examiner: Tran; Diem T
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
What is claimed is:
1. A device for purifying the exhaust gas of an internal combustion
engine comprising a particulate filter on which the trapped
particulates are oxidized and reversing means for reversing the
exhaust gas upstream side and the exhaust gas downstream side of
said particulate filter, wherein said particulate filter has a
trapping wall for trapping the particulates, said trapping wall has
a first trapping surface and a second trapping surface, said
reversing means reverses the exhaust gas upstream side and the
exhaust gas downstream side of said particulate filter so that said
first trapping surface and said second trapping surface are used
alternately to trap the particulates, said particulate filter has a
first opening portion and a second opening portion through which
the exhaust gas flows in and out from said particulate filter and
is arranged in the exhaust pipe upstream of the muffler, and at
least part of the circumferential portion of said particulate
filter between said first opening portion and said second opening
portion is contact with the flow of the exhaust gas in said exhaust
pipe.
2. A device for purifying the exhaust gas of an internal combustion
engine according to claim 1, wherein said trapping wall carries an
active-oxygen releasing agent, and active-oxygen released from said
active-oxygen releasing agent oxidizes the particulates.
3. A device for purifying the exhaust gas of an internal combustion
engine according to claim 2, wherein said active-oxygen releasing
agent takes in and holds oxygen when excessive oxygen is present in
the surroundings and releases the held oxygen as active-oxygen when
the oxygen concentration in the surroundings drops.
4. A device for purifying the exhaust gas of an internal combustion
engine according to claim 2, wherein said active-oxygen releasing
agent holds NO.sub.x to combine the NO.sub.x with oxygen when
excessive oxygen is present in the surroundings and releases to
decompose the combined NO.sub.x and oxygen into NO.sub.x and
active-oxygen when the oxygen concentration in the surroundings
drops.
5. A device for purifying the exhaust gas of an internal combustion
engine according to claim 1, wherein the center line passing
through said first opening portion and said second opening portion
of said particulate filter intersects with the center line of said
exhaust pipe in plan view.
6. A device for purifying the exhaust gas of an internal combustion
engine according to claim 5, wherein a plurality of said
particulate filters are arranged in the longitudinal direction of
said exhaust pipe.
7. A device for purifying the exhaust gas of an internal combustion
engine according to claim 5, wherein said exhaust pipe has a first
flow passage, a second flow passage, and a third flow passage that
are divided in the longitudinal direction, said first flow passage
is communicated with said first opening portion of said particulate
filter, said second flow passage is communicated with said second
opening portion of said particulate filter, and at least part of
said circumferential portion of said particulate filter is in
contact with the flow of the exhaust gas in said third flow
passage.
8. A device for purifying the exhaust gas of an internal combustion
engine according to claim 7, wherein said reversing means has a
valve body, makes said valve body to be a first position so that
said third flow passage is communicated with said first flow
passage and so that said second flow passage is communicated with
one of the exhaust gas upstream side and the exhaust gas downstream
side of said exhaust pipe, makes said valve body to be a second
position so that said third flow passage is communicated with said
second flow passage and so that said first flow passage is
communicated with said one of the exhaust gas upstream side and the
exhaust gas downstream side of said exhaust pipe, and changes over
said valve body from one of said first position and said second
position to the other so that the exhaust gas upstream side and the
exhaust gas downstream side of said particulate filter are
reversed.
9. A device for purifying the exhaust gas of an internal combustion
engine according to claim 2, wherein the center line passing
through said first opening portion and said second opening portion
of said particulate filter intersects with the center line of said
exhaust pipe in plan view.
10. A device for purifying the exhaust gas of an internal
combustion engine according to claim 3, wherein the center line
passing through said first opening portion and said second opening
portion of said particulate filter intersects with the center line
of said exhaust pipe in plan view.
11. A device for purifying the exhaust gas of an internal
combustion engine according to claim 4, wherein the center line
passing through said first opening portion and said second opening
portion of said particulate filter intersects with the center line
of said exhaust pipe in plan view.
12. A device for purifying the exhaust gas of an internal
combustion engine according to claim 6, wherein said exhaust pipe
has a first flow passage, a second flow passage, and a third flow
passage that are divided in the longitudinal direction, said first
flow passage is communicated with said first opening portion of
said particulate filter, said second flow passage is communicated
with said second opening portion of said particulate filter, and at
least part of said circumferential portion of said particulate
filter is in contact with the flow of the exhaust gas in said third
flow passage.
Description
TECHNICAL FIELD
The present invention relates to a device for purifying the exhaust
gas of an internal combustion engine.
BACKGROUND ART
The exhaust gas of an internal combustion engine and, particularly,
of a diesel engine, contains particulates comprising carbon as a
chief component. Particulates are harmful materials and thus it has
been suggested that a particulate filter should be arranged in the
exhaust system to trap particulates before they are emitted into
the atmosphere. In such a particulate filter, the trapped
particulates must be burned and removed to prevent resistance to
the exhaust gas from increasing due to the blocked meshes.
In such a regeneration of the particulate filter, if the
temperature of the particulates becomes about 600 degrees C., they
ignite and burn. However, usually, the temperature of an exhaust
gas of a diesel engine is considerably lower than 600 degrees C.
and thus a heating means is required to heat the particulate filter
itself.
Japanese Examined Patent Publication No. 7-106290 discloses that,
if one of the platinum group metals and one of the oxides of the
alkali earth metals are carried on the filter, the particulates on
the filter burn and are removed successively at about 400 degrees
C. 400 degrees C. is a typical temperature of the exhaust gas of a
diesel engine.
However, when the above-mentioned filter is used, the temperature
of the exhaust gas is not always about 400 degrees C. Further, a
large amount of particulates can be discharged from the diesel
engine according to an engine operating condition. Thus,
particulates that cannot be burned and removed each time can
deposit on the filter.
In this filter, if a certain amount of particulates deposits on the
filter, the ability to burn and remove particulates drops so much
that the filter cannot be regenerated by itself. Thus, if such a
filter is merely arranged in the exhaust system, the blocking of
the filter meshes can occur relative quickly.
DISCLOSURE OF THE INVENTION
Therefore, an object of the present invention is to provide a
device, for purifying the exhaust gas of an internal combustion
engine, which can oxidize and remove the trapped particulates on
the particulate filter, and can prevent blocking of the particulate
filter meshes.
According to the present invention, there is provided a device for
purifying the exhaust gas of an internal combustion engine
comprising a particulate filter on which the trapped particulates
are oxidized and removed and a reversing means for reversing the
exhaust gas upstream side and the exhaust gas downstream side of
the particulate filter, wherein the particulate filter has a
trapping wall for trapping the particulates, the trapping wall has
a first trapping surface and a second trapping surface, the
reversing means reverses the exhaust gas upstream side and the
downstream side of the particulate filter so that the first
trapping surface and the second trapping surf ace are used
alternately to trap the particulates, the particulate filter has a
first opening portion and a second opening portion through which
the exhaust gas flows in and out from the particulate filter, the
particulate filter is arranged in the exhaust pipe, and at least
part of the circumferential portion of the particulate filter
between the first opening portion and the second opening portion is
contact with the flow of the exhaust gas in the exhaust pipe.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic vertical sectional view of a diesel engine
with a device for purifying the exhaust gas according to the
present invention;
FIG. 2 is an enlarged vertical sectional view of the combustion
chamber of FIG. 1;
FIG. 3 is a bottom view of the cylinder-bead of FIG. 1;
FIG. 4 is a side sectional view of the combustion chamber;
FIG. 5 is a view showing the relationship between the lifting
amounts of the intake valve and the exhaust valve and the fuel
injection;
FIG. 6 is a view showing the amounts of produced smoke, NO.sub.x,
and the like;
FIG. 7(A) is a view showing the change in the combustion pressure
when the amount of produced smoke is the greatest near an air-fuel
ratio of 21;
FIG. 7(B) is a view showing the change in the combustion pressure
when the amount of produced smoke is substantially zero near an
air-fuel ratio of 18;
FIG. 8 is a view showing the fuel molecules;
FIG. 9 is a view showing the relationship between the amount of
produced smoke and the EGR rate;
FIG. 10 is a view showing the relationship between the amount of
injected fuel and the amount of mixed gas;
FIG. 11 is a view showing the first operating region (I) and the
second operating region (II);
FIG. 12 is a view showing the output of the air-fuel ratio
sensor;
FIG. 13 is a view showing the opening degree of the throttle valve
and the like;
FIG. 14 is a view showing the air-fuel ratio in the first operating
region (I);
FIG. 15(A) is a view showing the target opening degree of the
throttle valve;
FIG. 15(B) is a view showing the target opening degree of the EGR
control valve;
FIG. 16 is a view showing the air-fuel ratio in the second
operating region (II);
FIG. 17(A) is a view showing the target opening degree of throttle
valve;
FIG. 17(B) is a view showing the target opening degree of the EGR
control valve;
FIG. 18 is a plan sectional view showing the device for purifying
the exhaust gas of an internal combustion engine according to the
present invention;
FIG. 19 is a P--P sectional view of FIG. 18;
FIG. 20(A) is a front view showing the structure of the particulate
filter;
FIG. 20(B) is a side sectional view showing the structure of the
particulate filter;
FIGS. 21(A) and 21(B) are views explaining the oxidizing action of
the particulates;
FIG. 22 is a view showing the relationship between the amount of
particulates that can be oxidized and removed and the temperature
of the particulate filter;
FIGS. 23(A), 23(B), and 23(C) are views explaining the depositing
action of the particulates;
FIG. 24 is a first flowchart for preventing the deposition of the
particulates on the particulate filter;
FIGS. 25(A) and 25(B) are enlarged sectional views of the partition
wall of the particulate filter with the residual particulates;
FIG. 26 is a second flowchart for preventing the deposition of the
particulates on the particulate filter;
FIG. 27 is a third flowchart for preventing the deposition of the
particulates on the particulate filter; and
FIG. 28 is a plan sectional view showing a modification of the
device for purifying the exhaust gas of an internal combustion
engine of FIG. 18.
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a schematic vertical sectional view of a four-stroke
diesel engine with a device for purifying the exhaust gas according
to the present invention. FIG. 2 is an enlarged vertical sectional
view of a combustion chamber of diesel engine of FIG. 1. FIG. 3 is
a bottom view of a cylinder-head of diesel engine of FIG. 1.
Referring FIGS. 1-3, reference numeral 1 designates an engine body,
reference numeral 2 designates a cylinder-block, reference numeral
3 designates a cylinder-head, reference numeral 4 designates a
piston, reference numeral 5a designates a cavity formed on the top
surface of piston 4, reference numeral 5 designates a combustion
chamber formed in the cavity 5a, reference numeral 6 designates an
electrically controlled fuel injector, reference numeral 7
designates a pair of intake valves, reference numeral 8 designates
an intake port, reference numeral 9 designates a pair of exhaust
valves, and reference numeral 10 designates an exhaust port. The
intake port 8 is connected to a surge tank 12 via a corresponding
intake tube 11. The surge tank 12 is connected to an air-cleaner 14
via an intake duct 13. A throttle valve 16 driven by an electric
motor 15 is arranged in the intake duct 13. On the other hand, the
exhaust port 10 in connected to an exhaust manifold 17.
As shown in FIG. 1, an air-fuel ratio sensor 21 is arranged in the
exhaust manifold 17. The exhaust manifold 17 and the surge tank 12
are connected with each other via an EGR passage 22. An
electrically controlled EGR control valve 23 is arranged in the EGR
passage 22. An EGR cooler 24 is arranged around the EGR passage 22
to cool the EGR gas flowing in the EGR passage 22. In the
embodiment of FIG. 1, the engine cooling water is led into the EGR
cooler 24 and thus the EGR gas is cooled by the engine cooling
water.
On the other hand, each fuel injector 6 is connected to the fuel
reservoir, that is, a common rail 26 via a fuel supply tube 25.
Fuel is supplied to the common rail 26 from an electrically
controlled variable discharge fuel pump 27. Fuel supplied in the
common rail 26 is supplied to the fuel injector 6 via each fuel
supply tube 25. A fuel pressure sensor 28 for detecting a fuel
pressure in the common rail 26 is attached to the common rail 26.
The discharge amount of the fuel pump 27 is controlled on the basis
of an output signal of the fuel pressure sensor 28 such that the
fuel pressure in the common rail 26 becomes the target fuel
pressure.
Reference numeral 30 designates an electronic control unit. The
output signals of the air-fuel sensor 21 and the fuel pressure
sensor 28 are input thereto. An engine load sensor 41 is connected
to the accelerator pedal 40, which generates an output voltage
proportional to the amount of depression (L) of the accelerator
pedal 40. The output signal of the engine load sensor 41 is also
input to the electronic control unit 30. Further, the output signal
of a crank angle sensor 42 for generating an output pulse each time
the crankshaft rotates by, for example, 30 degrees is also input
thereto. Thus, the electronic control unit 30 actuates the fuel
injector 6, the electronic motor 15, the EGR control valve 23, and
the fuel pump 27 on the basis of the input signals.
As shown in FIGS. 2 and 3, in the embodiment of the present
invention, the fuel injector 6 comprises of a nozzle having six
nozzle holes. Fuel sprays (F) are injected from the nozzle holes in
slightly downward direction against a horizontal plane with equal
angular intervals. As shown in FIG. 3, two fuel sprays (F) of the
six fuel sprays (F) are scattered along the lower surface of each
exhaust valve 9. FIGS. 2 and 3 show the case where fuel is injected
at the end of the compression stroke. In this case, the fuel sprays
(F) progress toward the inside periphery surface of the cavity 5
and thereafter are ignited and burned.
FIG. 4 shows the case where additional fuel is injected from the
fuel injector 6 when the lifting amount of the exhaust valves 9 is
the maximum in the exhaust stroke. That is, FIG. 5 shows the case
where the main fuel injection (Qm) is carried out close to the
compression top dead center and thereafter the additional fuel
injection (Qa) is carried out in the middle stage of the exhaust
stroke. In this case, the fuel sprays (F) that progress toward the
exhaust valves 9 are directed between the umbrella-like back
surface of the exhaust valve 9 and the exhaust port 10. In other
words, two nozzle holes, of the six nozzle holes of the fuel
injector 6, are formed such that when the exhaust valves 9 are
opened and the additional fuel injection (Qa) is carried out, the
fuel sprays (F) are directed between the back surface of the
exhaust valve 9 and the exhaust port 10. In the embodiment of FIG.
4, these fuel sprays (F) impinge the back surface of the exhaust
valve 9 and are reflected from the back surface of the exhaust
valves 9, and thus are directed into the exhaust port 10.
Usually, the additional fuel injection (Qa) is not carried out, and
the main fuel injection (Qm) only is carried out. FIG. 6 indicates
an example of an experiment showing the change in the output torque
and the amount of smoke, HC, CO, and NO.sub.x exhausted at that
time when changing the air-fuel ratio A/F (abscissa in FIG. 6) by
changing the opening degree of the throttle valve 16 and the EGR
rate at the time of low engine load operation. As will be
understood from FIG. 6, in this experiment, the smaller the air
fuel ratio A/F becomes, the larger the EGR rate becomes. When the
air-fuel ratio is below the stoichiometric air-fuel ratio (nearly
equal 14.6), the EGR rate becomes over 65 percent.
As shown in FIG. 6, if the EGR rate is increased to reduce the
air-fuel ratio A/F, when the EGR rate becomes close to 40 percent
and the air-fuel ratio A/F becomes about 30, the amount of produced
smoke starts to increase. Next, when the EGR rate is further
increased and the air-fuel ratio A/F is made smaller, the amount of
produced smoke sharply increases and peaks. Next, when the EGR rate
is further increased and the air-fuel ratio A/F is made smaller,
the amount of produced smoke sharply decreases when the EGR rate is
made over 65 percent and the air-fuel ratio A/F becomes close to
15.0, the amount of produced smoke is substantially zero. That is,
almost no soot is produced. At this time, the output torque of the
engine falls somewhat and the amount of produced NO.sub.x becomes
considerably lower. On the other hand, at this time, the amounts of
produced HC and CO start to increase.
FIG. 7(A) shows the change in combustion pressure in the combustion
chamber S when the amount of produced smoke is the greatest near an
air-fuel ratio A/F of 21. FIG. 7(B) shows the change in combustion
pressure in the combustion chamber 5 when the amount of produced
smoke is substantially zero near an air-fuel ratio A/F of 18. As
will be understood from a comparison of FIG. 7(A) and FIG. 7(B),
the combustion pressure is lower in the case shown in FIG. 7(B)
where the amount of produced smoke is substantially zero than the
case shown in FIG. 7(A) where the amount of produced smoke is
large.
The following may be said from the results of the experiment shown
in FIGS. 6 and 7. That is, first, when the air-fuel ratio A/F is
less than 15.0 and the amount of produced smoke is substantially
zero, the amount of produced NO.sub.x decreases considerably as
shown in FIG. 6. The fact that the amount of produced NO.sub.x
decreases means that the combustion temperature in the combustion
chamber 5 falls. Therefore, it can be said that when almost no soot
is produced, the combustion temperature in the combustion chamber 5
becomes lower. The same fact can be said from FIG. 7. That is, in
the state shown in FIG. 7(B) where almost no soot is produced, the
combustion pressure becomes lower, therefore the combustion
temperature in the combustion chamber 5 becomes lower at this
time.
Second, when the amount of produced smoke, that is, the amount of
produced soot, becomes substantially zero, as shown in FIG. 6, the
amounts of exhausted HC and CO increase. This means that the
hydrocarbons are exhausted without changing into soot. That is, the
straight chain hydrocarbons and aromatic hydrocarbons contained in
the fuel and shown in FIG. 8 decompose when raised in temperature
in an oxygen insufficient state resulting in the formation of a
precursor of soot. Next, soot mainly composed of solid masses of
carbon atoms is produced. In is this case, the actual process of
production of soot is complicated. How the precursor of soot is
formed is not clear, but whatever the case, the hydrocarbons shown
in FIG. 8 change to soot through the soot precursor. Therefore, as
explained above, when the amount of production of soot becomes
substantially zero, the amount of exhaust of HC and Co increases as
shown in FIG. 6, but the HC at this time is a soot precursor or in
a state of hydrocarbon before that.
Summarizing these considerations based on the results of the
experiments shown in FIGS. 6 and 7, when the combustion temperature
in the combustion chamber 5 is low, the amount of produced soot
becomes substantially zero. At this time, a soot precursor or a
state of hydrocarbons before that is exhausted from the combustion
chamber 5. More detailed experiments and studies were conducted. As
a result, it was learned that when the temperature of the fuel and
the gas around the fuel in the combustion chamber 5 is below a
certain temperature, the process of growth of soot stops midway,
that is, no soot at all is produced and that when the temperature
of the fuel and the gas around the fuel in the combustion chamber 5
becomes higher than the certain temperature, soot is produced.
The temperature of the fuel and the gas around the fuel when the
process of growth of hydrocarbons stops in the state of the soot
precursor, that is, the above certain temperature, changes
depending on various factors such as the type of the fuel, the
air-fuel ratio, and the compression ratio, so it cannot be said
what degree it is, but this certain temperature is deeply related
to the amount of production of NO.sub.x. Therefore, this certain
temperature can be defined to a certain degree from the amount of
production of NO.sub.x. That is, the greater the EGR rate is, the
lower the temperature of the fuel, and the gas around it at the
time of combustion, becomes and the lower the amount of produced
NO.sub.x becomes. At this time, when the amount of produced
NO.sub.x becomes around 10 ppm or less, almost no soot is produced
any more. Therefore, the above certain temperature substantially
corresponds to the temperature when the amount of produced NO.sub.x
becomes around 10 ppm or less.
Once soot is produced, it is impossible to purify it by
after-treatment using a catalyst having an oxidation function. As
opposed to this, a soot precursor or a state of hydrocarbons before
that can be easily purified by after-treatment using a catalyst
having an oxidation function. Thus, it is extremely effective for
the purifying of the exhaust gas that the hydrocarbons are
exhausted from the combustion chamber 5 in the form of a soot
precursor or a state before that with the reduction of the amount
of produced NO.sub.x.
Now, to stop the growth of hydrocarbons in the state before the
production of soot, it is necessary to suppress the temperature of
the fuel and the gas around it at the time of combustion in the
combustion chamber 5 to a temperature lower than the temperature
where soot is produced. In this case, it was learned that the heat
absorbing action of the gas around the fuel at the time of
combustion of the fuel has an extremely great effect in suppression
the temperatures of the fuel and the gas around it.
That is, if only air exists around the fuel, the vaporized fuel
will immediately react with the oxygen in the air and burn. In this
case, the temperature of the air away from the fuel does not rise
so much. Only the temperature around the fuel becomes locally
extremely high. That is, at this time, the air away from the fuel
does not absorb the heat of combustion of the fuel much at all. In
this case, since the combustion temperature becomes extremely high
locally, the unburned hydrocarbons receiving the heat of combustion
produce soot.
On the other hand, when fuel exists in a mixed gas of a large
amount of inert gas and a small amount of air, the situation is
somewhat different. In this case, the evaporated fuel disperses in
the surroundings and reacts with the oxygen mixed in the inert gas
to burn. In this case, the heat of combustion is absorbed by the
surrounding inert gas, so the combustion temperature no longer
rises so much. That is, the combustion temperature can be kept low.
That is, the presence of inert gas plays an important role in the
suppression of the combustion temperature. It is possible to keep
the combustion temperature low by the heat absorbing action of the
inert gas.
In this case, to suppress the temperature of the fuel and the gas
around it to a temperature lower than the temperature at which soot
is produced, an amount of inert gas enough to absorb an amount of
heat sufficient for lowering the temperatures is required.
Therefore, if the amount of fuel increases, the amount of required
inert gas increases with this. Note that, in this case, the larger
the specific heat of the inert gas is, the stronger the heat
absorbing action becomes. Therefore, a gas with a large specific
heat is preferable as the inert gas. In this regard, since CO.sub.2
and EGR gas have relatively large specific heats, it may be said to
be preferable to use EGR gas as the inert gas.
FIG. 9 shows the relationship between the EGR rate and smoke when
using EGR gas as the inert gas and changing the degree of cooling
of the EGR gas. That is, the curve (A) in FIG. 9 shows the case of
strongly cooling the EGR gas and maintaining the temperature of the
EGR gas at about 90 degrees C., the curve (B) shows the case of
cooling the EGR gas by a compact cooling apparatus, and the curve
(C) shows the case of not compulsorily cooling the EGR gas.
When strongly cooling the EGR gas as shown by the curve (A) in FIG.
9, the amount of produced soot peaks when the EGR rate is a
slightly below 50 percent. In this case, if the EGR rate is made
about 55 percent or higher, almost no soot is produced any longer.
On the other hand, when the EGR gas is slightly cooled as shown by
the curve (B) in FIG. 9, the amount of produced soot peaks when the
EGR rate is slightly higher than 50 percent. In this case, if the
EGR rate is made above about 65 percent, almost no soot is
produced.
Further, when the EGR gas is not forcibly cooled as shown by the
curve (C) in FIG. 9, the amount of produced soot peaks near an EGR
rate of 55 percent. In this case, if the EGR rate is made over
about 70 percent, almost no soot is produced. Note that FIG. 9
shows the amount of produced smoke when the engine load is
relatively high. When the engine load becomes smaller, the EGR rate
at which the amount of produced soot peaks falls somewhat, and the
lower limit of the EGR rate, at which almost no soot is produced,
also falls somewhat. In this way, the lower limit of the EGR rate
at which almost no soot is produced changes in accordance with the
degree of cooling of the EGR gas or the engine load.
FIG. 10 shows the amount of mixed gas of EGR gas and air, the ratio
of air in the mixed gas, and the ratio of EGR gas in the mixed gas,
required to make the temperature of the fuel and the gas around it,
at the time of combustion, a temperature lower than the temperature
at which soot is produced in the case of the use of EGR gas as an
inert gas. Note that, in FIG. 10, the ordinate shows the total
amount of suction gas taken into the combustion chamber 5. The
broken line (Y) shows the total amount of suction gas able to be
taken into the combustion chamber 5 when supercharging is not being
performed. Further, the abscissa shows the required load. (Z1)
shows the low engine load operation region,
Referring to FIG. 10, the ratio of air, that is, the amount of air
in the mixed gas shows the amount of air necessary for causing the
injected fuel to completely burn. That is, in the case shown in
FIG. 10, the ratio of the amount of air and the amount of injected
fuel becomes the stoichiometric air-fuel ratio. On the other hand,
in FIG. 10, the ratio of EGR gas, that is, the amount of EGR gas in
the mixed gas, shows the minimum amount of EGR gas required for
making the temperature of the fuel and the gas around it a
temperature lower than the temperature at which soot is produced
when the injected fuel has burned completely. This amount of EGR
gas is, expressed in term of the EGR rate, equal to or larger than
55 percent, in the embodiment shown in FIG. 10, it is equal to or
larger than 70 percent. That is, if the total amount of suction gas
taken into the combustion chamber 5 is made the solid line (X) in
FIG. 10 and the ratio between the amount of air and the amount of
EGR gas in the total amount of suction gas (X) is made the ratio
shown in FIG. 10, the temperature of the fuel and the gas around it
becomes a temperature lower than the temperature at which soot is
produced and therefore no soot at all is produced. Further, the
amount of produced NO.sub.x at this time is about 10 ppm or less
and therefore the amount of produced NO.sub.x becomes extremely
small.
If the amount of injected fuel increases, the amount of generated
heat at the time of combustion increases, so to maintain the
temperature of the fuel and the gas around it at a temperature
lower than the temperature at which soot is produced, the amount of
heat absorbed by the EGR gas must be increased. Therefore, as shown
in FIG. 10, the amount of EGR gas has to be increased with an
increase in the amount of injected fuel. That is, the amount of EGR
gas has to be increased as the required engine load becomes
higher.
On the other hand, in the engine load region (Z2) of FIG. 10, the
total amount of suction gas (X) required for inhibiting the
production of soot exceeds the total amount of suction gas (Y) that
can be taken in. Therefore, in this case, to supply the total
amount of suction gas (x), required for inhibiting the production
of soot, into the combustion chamber 5, it is necessary to
supercharge or pressurize both the EGR gas and the intake air or
just the EGR gas. When not supercharging or pressurizing the EGR
gas etc., in the engine load region (Z2), the total amount of
suction gas (X) corresponds to the total amount of suction gas (Y)
that can be taken in. Therefore, in this case, to inhibit the
production of soot, the amount of air is reduced somewhat to
increase the amount of EGR gas and the fuel is made to burn in a
state where the air-fuel ratio is rich.
As explained above, FIG. 10 shows the case of combustion of fuel at
the stoichiometric air-fuel ratio. In the low engine load operating
region (Z1) shown in FIG. 10, even if the amount of air is made
smaller than the amount of air shown in FIG. 10, that is, even if
the air-fuel ratio is made rich, it is possible to inhibit the
production of soot and make the amount of produced NO.sub.x around
10 ppm or less. Further, in the low engine load operating region
(Z1) shown in FIG. 10, even of the amount of air is made greater
than the amount of air shown in FIG. 10, that is, the average of
air-fuel ratio is made lean at 17 to 18, it is possible to inhibit
the production of soot and make the amount of produced NO.sub.x
around 10 ppm or less.
That is, when the air-fuel ratio is made rich, the fuel is in
excess, but since the combustion temperature is suppressed to a low
temperature, the excess fuel does not change into soot and
therefore soot is not produced. Further, at this time, only an
extremely small amount of NO.sub.x is produced. On the other hand,
when the average of air-fuel ratio is lean or when the air-fuel
ratio is the stoichiometric air-fuel ratio, a small amount of soot
is produced it the combustion temperature becomes higher, but the
combustion temperature is suppressed to a low temperature, and thus
no soot at all is produced. Further, only an extremely small amount
of NO.sub.x is produced.
In this way, in the low engine load operating region (Z1), despite
the air-fuel ratio, that is, whether the air fuel ratio is rich or
the stoichiometric air-fuel ratio, or the average air-fuel ratio is
lean, no soot is produced and the amount of produced NO.sub.x
becomes extremely small. Therefore, considering the improvement of
the fuel consumption rate, it may be said to be preferable to make
the average of air-fuel ratio lean.
By the way, only when the engine load is relative low and the
amount of generated heat is small, can the temperature of the fuel
and the gas around the fuel in the combustion be suppressed to
below a temperature at which the process of growth of soot stops
midway. Therefore, in the embodiment of the present invention, when
the engine load is relative low, the temperature of the fuel and
the gas around the fuel in the combustion is suppressed to below a
temperature at which the process of growth of soot stops midway and
thus a first combustion, i.e., a low temperature combustion is
carried out. When the engine load is relative high, a second
combustion, i.e., normal combustion as usual is carried out. Here,
as can be understood from the above explanation, the first
combustion, i.e., the low temperature combustion is a combustion in
which the amount of inert gas in the combustion chamber is larger
than the worst amount of inert gas causing the maximum amount of
produced soot and thus no soot at all is produced. The second
combustion, i.e., the normal combustion is a combustion in which
the amount of inert gas in the combustion chamber is smaller than
the worst amount of inert gas.
FIG. 11 shows a first operating region (I) in which the first
combustion, i.e., the low temperature combustion is carried out and
a second operating region (II) in which the second combustion,
i.e., the normal combustion is carried out. In FIG. 11, the
ordinate (L) shows the amount of depression of the accelerator
pedal 40, i.e., the required engine load. The abscissa (N) shows
the engine speed. Further, in FIG. 11, X(N) shows a first boundary
between the first operating region (I) and the second operating
region (II). Y(N) shows a second boundary between the first
operating region (I) and the second operating region (II). The
decision of changing from the first operating region (I) to the
second operating region (II) is carried out on the basis of the
first boundary X(N). The decision of changing from the second
operating region (II) to the first operating region (I) is carried
out on the basis of the second boundary Y(N).
That is, when the engine operating condition is in the first
operating region (I) and the low temperature combustion is carried
out, if the required engine load (L) increases beyond the first
boundary X(N) that is a function of the engine speed (N), it is
determined that the engine operating region shifts in the second
operating region (II) and thus the normal combustion is carried
out. Thereafter, if the required engine load (L) decreases below
the second boundary Y(N) that is a function of the engine speed
(N), it is determined that the engine operating region shifts in
the first operating region (I) and thus the low temperature
combustion is carried out again.
FIG. 12 shows the output of the air-fuel ratio sensor 21. As shown
in FIG. 12, the output current (I) of the air-fuel ratio sensor 21
changes in accordance with the air-fuel ratio A/F. Accordingly, the
air-fuel ratio can be known from the output current (I) of the
air-fuel ration sensor 21. Next, referring FIG. 13, the engine
operating control in the first operating region (I) and the second
operating region (II) will be explained schematically.
FIG. 13 shows the opening degree of the throttle valve 16, the
opening degree of the EGR control valve 23, the EGR rate, the
air-fuel ratio, the fuel injection timing, and the amount of
injected fuel with respect to the required engine load (L). As
shown in FIG. 13, in the first operating region (I) when the
required engine load (L) is low, the throttle valve 16 is gradually
opened from near the fully closed state to near the half opened
state along with an increase in the required engine load (L), and
the EGR control valve 23 is gradually opened from near the fully
closed state to the fully opened state along with an increase in
the required engine load (L). In the embodiment shown in FIG. 13,
the EGR rate in the first operating region (I) is made about 70
percent and the air-fuel ratio therein is made slightly lean.
In the other words, in the first operating region (I), the opening
degrees of the throttle valve 16 and the EGR control valve 23 are
controlled such that the EGR rate becomes about 70 percent and the
air-fuel ratio becomes a slightly lean air-fuel ratio. The air-fuel
ratio at this time is controlled to the target air-fuel ratio to
correct the opening degree of the EGR control valve 23 on the basis
of the output signal of the air-fuel ratio sensor 21. In the first
operating region (I), the fuel is injected before the compression
top dead center TDC. In this case, the starting time (.theta.S) of
fuel injection is delayed along with an increase in the required
engine load (L) and the ending time (.theta.E) of fuel injection is
delayed along with the delay of the starting time (.theta.S) of
fuel injection.
When in the idle operating mode, the throttle valve 16 is closed to
near the fully closed state. In this time, the EGR control valve 23
is also closed near the fully closed state. When the throttle valve
16 is closed to near the fully closed state, the pressure in the
combustion chamber 5 in the initial stage of the compression stroke
is made low and thus the compression pressure becomes low. When the
compression pressure becomes low, the compression work of the
piston 4 becomes small and thus the vibration of the engine body 1
becomes small. That is, when in the idle operating mode, the
throttle valve 16 is closed near the fully closed state to restrain
the vibration of the engine body 1.
On the other hand, when the engine operating region changes from
the first operating region (I) to the second operating region (II),
the opening degree of the throttle valve 16 increases by a step
from the half opened state toward the fully opened state. In this
time, in the embodiment shown in FIG. 13, the EGR rate decreases by
a step from about 70 percent to below 40 percent and the air-fuel
ratio increases by a step. That is, the EGR rate jumps beyond the
EGR rate extent (FIG. 9) in which the large amount of smoke is
produced and thus a large amount of smoke is not produced when the
engine operating region changes from the first operating region (I)
to the second operating region (II).
In the second operating region (II), the normal combustion as usual
is carried out. This combustion causes some production of soot and
NO.sub.x. However, the thermal efficiency thereof is higher than
that of the low temperature combustion. Thus, when the engine
operating region changes from the first operating region (I) to the
second operating region (II), the amount of injected fuel decreases
by a step as shown in FIG. 13.
In the second operating region (II), the throttle valve 16 is held
in the fully opened state except in a part thereof. The opening
degree of the EGR control valve 23 decreases gradually along with
an increase in the required engine load (L). In this operating
region (II), the EGR rate decreases along with an increase in the
required engine load (L) and the air-fuel ratio decreases along
with an increase in the required engine load (L). However, the
air-fuel ratio is made a lean air-fuel ratio even it the required
engine load (L) becomes high. Further, in the second operating
region (II), the starting time (.theta.S) of fuel injection is made
near the compression top dead center TDC.
FIG. 14 shows the air-fuel ratios A/F in the first operating region
(I). In FIG. 14, the curves indicated by A/F=15.5, A/P=16, A/F=17,
and A/F=18 shows respectively the cases in that the air-fuel ratios
are 15.5, 16, 17, and 18. The air-fuel ratio between two of the
curves is defined by the proportional allotment. As shown in FIG.
14, in the first operating region (I), the air-fuel ratio is lean
and the lower the required engine load (L) becomes, the more the
air-fuel ratio is lean.
That is, the amount of generated heat in the combustion decreases
along with a decrease in the required engine load (L). Therefore,
even if the EGR rate decreases along with a decrease in the
required engine load (L), the low temperature combustion can be
carried out. When the EGR rate decreases, the air-fuel ratio
becomes large. Therefore, as shown in FIG. 14, the air-fuel ratio
A/F increases along with a decrease in the required engine load
(L). The larger the air-fuel ratio becomes, the more the fuel
consumption improves. Accordingly, in the present embodiment, the
air-fuel ratio A/F increases along with a decrease in the required
engine load (L) such that the air-fuel ratio is made lean as much
as possible.
A target opening degree (ST) of the throttle valve 16 required to
make the air-fuel ratio the target air-fuel ratio shown in FIG. 14
is memorized in ROM of the electronic control unit as a map in
which it is a function of the required engine load (L) and the
engine speed (N) shown in FIG. 15(A). A target opening degree (SE)
of the EGR control valve 23 required to make the air-fuel ratio the
target air-fuel ratio shown in FIG. 14 is memorized in ROM of the
electronic control unit as a map in which it is a function of the
required engine load (L) and the engine speed (N) shown in FIG.
15(B).
FIG. 16 shows target air-fuel ratios when the second combustion,
i.e., the normal combustion as usual is carried out. In FIG. 16,
the curves indicated by A/F=24, A/F=35, A/F=45, and A/F=60 shows
respectively the cases in that the target air-fuel ratios are 24,
35, 45, and 60. A target opening degree (ST) of the throttle valve
16, required to make the air-fuel ratio the target air-fuel ratio,
is memorized in the ROM of the electronic control unit as a map in
which it is a function of the required engine load (L) and the
engine speed (N) shown in FIG. 17(A). A target opening degree (SE)
of the EGR control valve 23 required to make the air-fuel ratio the
target air-fuel ratio is memorized in ROM of the electronic control
unit as a map in which it is a function of the required engine load
(L) and the engine speed (N) shown in FIG. 17(B).
Thus, in the diesel engine of the present embodiment, the first
combustion, i.e., the low temperature combustion and the second
combustion, i.e., the normal combustion are changed over on the
basis of the amount of depression (L) of the accelerator pedal 40
and the engine speed (N). In each combustion, the opening degrees
of the throttle valve 16 and the EGR control valve are controlled
by the maps shown in FIGS. 15 and 17 on the basis of the amount of
depression (L) of the accelerator pedal 40 and the engine speed
(N).
FIG. 18 is a plan view illustrating a device for purifying the
exhaust gas, and FIG. 19 is a P--P sectional view of FIG. 18. The
device is connected to the immediate downstream side of the exhaust
manifold 17 and is thus positioned in the exhaust system
considerably upstream of the muffler that is positioned at the
atmosphere opening end. The device has an exhaust pipe 71, The
exhaust pipe 71 has a first small diameter portion 71a that is
connected to the exhaust manifold 17, a second small diameter
portion 71b that is connected to the downstream exhaust passage,
and a large diameter portion 71c that positions between the first
small diameter portion 71a and the second small diameter portion
71b. Both ends of the large diameter portion 71c have a frustum
shape and are connected to the first diameter portion 71a and the
second diameter portion 71b.
The inside space of the large diameter portion 71c is divided by
two wall portions 72a, 72b that extend in the longitudinal
direction and that are parallel each other, and thus forms a first
flow passage 73a and a second flow passage 73b that position at
both sides, and a third flow passage 73c that positions at the
center. Here, all the exhaust gas from the first small diameter
portion 71a flows in the third flow passage 73c. In the large
diameter portion 71c, a particulate filter 70 that has a front
shape of an oval is arranged such that the center lines of the
particulate filter and the large diameter portion 71c intersect at
right angles and such that the particulate filter 70 penetrates
through both of the two wall portions 71a and 71b.
The particulate filter 70 has a first opening portion 70a and a
second opening portion 70b through which the exhaust gas flows in
and out from the particulate filter. The first opening portion 70a
communicates with the first flow passage 73a and the second opening
portion 70b communicates with the second flow passage 73b. Here,
the above center line of the particulate filter means the center
line that passes through the first opening portion 70a and the
second opening portion 70b. As explained below in detail, each of
the first opening portion 70a and the second opening portion 70b of
the particulate filter 70 is constructed from a plurality of the
openings. The third flow passage 73c is partly divided into the
upper part and the lower part by the circumferential portion of the
particulate filter 70 between the first opening portion 70a and the
second opening portion 70b. Thus, the exhaust gas flows in contact
with the circumferential portion 70c of the particulate filter 70
in the third flow passage.
Further, a pivotable valve body 74 is arranged at the downstream
end of the large diameter portion 71c. In a first position of the
valve body 74 shown by the solid-line, the third flow passage 73c
is communicated with the first flow passage 73a and the second flow
passage 73b is communicated with the second small diameter portion
71b. Therefore, the exhaust gas flows from the third flow passage
73c to the first flow passage 73a, and passes through the
particulate filter 70 from the first opening portion 70a to the
second opening portion 70b as shown by the solid-line arrow, and
flows out to the second small diameter portion 71b via the second
flow passage 73b. In a second position of the valve body 74 shown
by the chain-line, the third flow passage 73c is communicated with
the second flow passage 73b and the first flow passage 73a is
communicated with the second small diameter portion 71b. Therefore,
the exhaust gas flows from the third flow passage 73c to the second
flow passage 73b, and passes through the particulate filter 70 from
the second opening portion 70b to the first opening portion 70a as
shown by the chain-line arrow, and flows out to the second small
diameter portion 71b via the first flow passage 73a.
Thus, the valve body 74 is changed over from one of the first
position and the second position to the other so that the upstream
side and the downstream side of the particulate filter 70 can be
reversed. Further, if the valve body 74 is assumed at an opening
position between the first position and the second position, the
exhaust gas flows from the third flow passage 73c to the second
small diameter portion 71b without passing through the particulate
filter 70 and thus the exhaust gas can bypass the particulate
filter 70.
Thus, the present device for purifying the exhaust gas can reverse
the exhaust gas upstream side and the exhaust gas downstream side
of the particulate filter by a very simple structure. Further, the
particulate filter requires a large opening area to facilitate the
introduction of the exhaust gas. In the device, the opening
portions of the particulate filter is arranged in the longitudinal
direction of the exhaust pipe and thus the particulate filter
having a large opening area can be used without making it difficult
to mount it on the vehicle.
FIG. 20 shows the structure of the particulate filter 70, wherein
(A) is a front view of the particulate filter 70, i.e., a view
showing FIG. 19 from the direction of arrow, and (B) is a side
sectional view thereof. As shown in these figures, the particulate
filter 70 has an oval shape, and is, for example, the wall-flow
type of a honeycomb structure formed of a porous material such as
cordierite, and has many spaces in the axial direction divided by
many partition walls 54 extending in the axial direction. One of
any two neighboring spaces is closed by a plug 53 on the exhaust
gas downstream side, and the other one is closed by a plug 53 on
the exhaust gas upstream side. Thus, one of the two neighboring
spaces serves as an exhaust gas flow-in passage 50 and the other
one serves as an exhaust gas flow-out passage 51, causing the
exhaust gas to necessarily pass through the partition wall 54 as
indicated by arrows in FIG. 20(B). The particulates contained in
the exhaust gas are much smaller than the pores of the partition
wall 54, but collide with and are trapped on the exhaust gas
upstream side surface of the partition wall 54 and the pores
surface in the partition wall 54. Thus, each partition wall 54
works as a trapping wall for trapping the particulates. In the
present particulate filter 70, in order to oxidize and remove the
trapped particulates, an active-oxygen releasing agent and a noble
metal catalyst, which will be explained below, are carried on both
side surfaces of the partition wall 54 and preferably also on the
pore surfaces in the partition wall 54.
The active-oxygen releasing agent releases active-oxygen to promote
the oxidation of the particulates and, preferably, takes in and
holds oxygen when excessive oxygen is present in the surroundings
and releases the held oxygen as active-oxygen when the oxygen
concentration in the surroundings drops.
As the noble metal catalyst, platinum Pt is usually used. As the
active-oxygen releasing agent, there is used at least one selected
from alkali metals such as potassium K, sodium Na, lithium Li,
cesium Cs, and rubidium Rb, alkali earth metals such as barium Ba,
calcium Ca, and strontium Sr, rare earth elements such as lanthanum
La and yttrium Y, and transition metals.
As an active-oxygen releasing agent, it is desired to use an alkali
metal or an alkali earth metal having an ionization tendency
stronger than that of calcium Ca, i.e., to use potassium K, lithium
Li, cesium Cs, rubidium Rb, barium Ba, or strontium Sr.
Next, explained below is how the trapped particulates on the
particulate filter are oxidized and removed by the particulate
filter carrying such an active-oxygen releasing agent with
reference to the case of using platinum Pt and potassium K. The
particulates are oxidized and removed in the same manner even by
using another noble metal and another alkali metal, an alkali earth
metal, a rear earth element, or a transition metal.
In a diesel engine, the combustion usually takes place in an excess
air condition and, hence, the exhaust gas contains a large amount
of excess air. That is, if the ratio of the air to the fuel
supplied to the intake system and to the combustion chamber is
referred to as an air-fuel ratio of the exhaust gas, the air-fuel
ratio is lean. Further, NO generates in the combustion chamber and,
hence, the exhaust gas contains NO. Further, the fuel contains
sulfur S and sulfur S reacts with oxygen in the combustion chamber
to form SO.sub.2. Accordingly, the exhaust gas contains SO.sub.2.
Therefore, the exhaust gas containing excessive oxygen, NO, and
SO.sub.2 flows into the exhaust gas upstream side of the
particulate filter 70.
FIGS. 21(A) and 21(B) are enlarged views schematically illustrating
the surface of the particulate filter 70 with which the exhaust gas
comes in contact. In FIGS. 21(A) and 21(B), reference numeral 60
denotes a particle of platinum Pt and 61 denotes the active-oxygen
releasing agent containing potassium K.
As described above, the exhaust gas contains a large amount of
excess oxygen. When the exhaust gas contacts with the exhaust gas
contact surface of the particulate filter, oxygen O.sub.2 adheres
onto the surface of platinum Pt in the form of O.sub.2.sup.- or
O.sup.2- as shown in FIG. 21(A). On the other hand, NO in the
exhaust gas reacts with O.sub.2.sup.- or O.sup.2- on the surface of
platinum Pt to produce NO.sub.2 (2NO+O.sub.2.fwdarw.2NO.sub.2).
Next, a part of the produced NO.sub.2 is absorbed in the
active-oxygen releasing agent 61 while being oxidized on platinum
Pt, and diffuses in the active-oxygen releasing agent 61 in the
form of nitric acid ion NO.sub.3.sup.- while being combined with
potassium K to form potassium nitrate KNO.sub.3 as shown in FIG.
21(A). Thus, in the present embodiment, NO.sub.x contained in the
exhaust gas is absorbed in the particulate filter 70 and an amount
thereof released into the atmosphere can be decreased.
Further, the exhaust gas contains SO.sub.2, as described above, and
SO.sub.2 also is absorbed in the active-oxygen releasing agent 61
due to a mechanism similar to that of the case of NO. That is, as
described above, oxygen O.sub.2 adheres on the surface of platinum
Pt in the form of O.sub.2.sup.- or O.sup.2-, and SO.sub.2 in the
exhaust gas reacts with O.sub.2.sup.- or O.sup.2- on the surface of
platinum Pt to produce SO.sub.3. Next, a part of the produced
SO.sub.3 is absorbed in the active-oxygen releasing agent 61 while
being oxidized on the platinum Pt and diffuses in the active-oxygen
releasing agent 61 in the form of sulfuric acid ion SO.sub.4.sup.2-
while being combined with potassium K to produce potassium sulfate
K.sub.2 SO.sub.4. Thus, potassium nitrate KNO.sub.3 and potassium
sulfate K.sub.2 SO.sub.4 are produced in the active-oxygen
releasing agent 61.
The particulates in the exhaust gas adhere on the surface of the
active-oxygen releasing agent 61 carried by the particulate filter
as designated at 62 in FIG. 21(B). At this time, the oxygen
concentration drops on the surface of the active-oxygen releasing
agent 61 with which the particulates 62 are in contact. As the
oxygen concentration drops, there occurs a difference in the
concentration from the active-oxygen releasing agent 61 having a
high oxygen concentration and, thus, oxygen in the active-oxygen
releasing agent 61 tends to migrate toward the surface of the
active-oxygen releasing agent 61 with which the particulates 62 are
in contact. As a result, potassium nitrate KNO.sub.3 produced in
the active-oxygen releasing agent 61 is decomposed into potassium
K, oxygen O and NO, whereby oxygen O migrates toward the surface of
the active-oxygen releasing agent 61 with which the particulates 62
are in contact, and NO is emitted to the outside from the
active-oxygen releasing agent 61. NO emitted to the outside is
oxidized on the platinum Pt on the downstream side and is absorbed
again in the active-oxygen releasing agent 61.
At this time, further, potassium sulfate K.sub.2 SO.sub.4 produced
in the active-oxygen releasing agent 61 is also decomposed into
potassium K, oxygen O, and SO.sub.2, whereby oxygen O migrates
toward the surface of the active-oxygen releasing agent 61 with
which the particulates 62 are in contact, and SO.sub.2 is emitted
to the outside from the active-oxygen releasing agent 61. SO.sub.2
released to the outside is oxidized on the platinum Pt on the
downstream side and is absorbed again in the active-oxygen
releasing agent 61. Here, however, potassium sulfate K.sub.2
SO.sub.4 is stable and releases less active-oxygen than potassium
nitrate KNO.sub.3.
On the other hand, oxygen O migrating toward the surface of the
active-oxygen releasing agent 61 with which the particulates 62 are
in contact is the one decomposed from such compounds as potassium
nitrate KNO.sub.3 or potassium sulfate K.sub.2 SO.sub.4. Oxygen O
decomposed from the compound has a high level of energy and
exhibits a very high activity. Therefore, oxygen migrating toward
the surface of the active-oxygen releasing agent 61 with which the
particulates 62 are in contact is active-oxygen O. Upon coming into
contact with active-oxygen O, the particulates 62 are oxidized
without producing luminous flame in a short time, for example, a
few minutes or a few tens of minutes. Further, active-oxygen to
oxidize the particulates 62 is also released when NO and SO.sub.2
have been absorbed in the active-oxygen releasing agent 61. That
is, it can be considered that NO.sub.x diffuses in the
active-oxygen releasing agent 61 in the form of nitric acid ion
NO.sub.3.sup.- while being combined with oxygen atoms and to be
separated from oxygen atoms, and during this time, active-oxygen is
produced. The particulates 62 are also oxidized by this
active-oxygen. Further, the particulates 62 adhered on the
particulate filter 70 are not oxidized only by active-oxygen, but
also by oxygen contained in the exhaust gas.
The higher the temperature of the particulate filter becomes, the
more the platinum Pt and the active-oxygen releasing agent 61 are
activated. Therefore, the higher the temperature of the particulate
filter becomes, the larger the amount of active-oxygen O released
from the active-oxygen releasing agent 61 per unit time becomes.
Further, naturally, the higher the temperature of particulates is,
the easier the particulates are oxidized. Therefore, the amount of
particulates that can be oxidized and removed without producing
luminous flame on the particulate filter per unit time increases
with along an increase in the temperature of the particulate
filter.
The solid line in FIG. 22 shows the amount of particulates (G) that
can be oxidized and removed without producing luminous flame per
unit time. In FIG. 22, the abscissa represents the temperature TF
of the particulate filter. Here, FIG. 22 shows the case that the
unit time is 1 second, that is, the amount of particulates (G) that
can be oxidized and removed per 1 second. However, any time such as
1 minute, 10 minutes, or the like can be selected as unit time. For
example, in the case that 10 minutes is used as unit time, the
amount of particulates (G) that can be oxidized and removed per
unit time represents the amount of particulates (G) that can be
oxidized and removed per 10 minutes. In also this case, the amount
of particulates (G) that can be oxidized and removed without
producing luminous flame increases along with an increase in the
temperature of particulate filter 70 as shown in FIG. 22.
The amount of particulates emitted from the combustion chamber per
unit time is referred to as an amount of emitted particulates (M).
When the amount of emitted particulates (M) is smaller than the
amount of particulates (G) that can be oxidized and removed, for
example, the amount of emitted particulates (M) per 1 second is
smaller than the amount of particulates (G) that can be oxidized
and removed per 1 second or the amount of emitted particulates (M)
per 10 minutes is smaller than the amount of particulates (G) that
can be oxidized and removed per 10 minutes, that is, in the area
(I) of FIG. 22, the particulates emitted from the combustion
chamber are all oxidized and removed without producing luminous
flame successively on the particulate filter 70 for the short
time.
On the other hand, when the amount of emitted particulates (M) is
larger than the amount of particulates that can be oxidized and
removed (G), that is, in the area (II) of FIG. 22, the amount of
active-oxygen is not sufficient for all particulates to be oxidized
and removed successively. FIGS. 23(A) to (C) illustrate the manner
of oxidation of the particulates in such a case.
That is, in the case that the amount of active-oxygen is lacking
for oxidizing all particulates, when the particulates 62 adhere on
the active-oxygen releasing agent 61, only a part of the
particulates is oxidized as shown in FIG. 23(A), and the other part
of the particulates that was not oxidized sufficiently remains on
the exhaust gas upstream surface of the particulate filter. When
the state where the amount of active-oxygen is lacking continues, a
part of the particulates that was not oxidized remains on the
exhaust gas upstream surface of the particulate filter
successively. As a result, the exhaust gas upstream surface of the
particulate filter is covered with the residual particulates 63 as
shown in FIG. 23(B).
The residual particulates 63 are gradually transformed into
carbonaceous matter that can hardly be oxidized. Further, when the
exhaust gas upstream surface is covered with the residual
particulates 63, the action of the platinum Pt for oxidizing NO and
SO.sub.2, and the action of the active-oxygen releasing agent 61
for releasing active-oxygen are suppressed. The residual
particulates 63 can be gradually oxidized over a relative long
period. However, as shown in FIG. 23(C), other particulates 64
deposit on the residual particulates 63 one after the other, and
when the particulates are deposited so as to laminate, even if they
are the easily oxidized particulates, these particulates may not be
oxidized since these particulates are separated from the platinum
Pt or from the active-oxygen releasing agent. Accordingly, other
particulates deposit successively on these particulates 64. That
is, when the state where the amount of emitted particulates (M) is
larger than the amount of particulates that can be oxidized and
removed (G) continues, the particulates deposit to laminate on the
particulate filter.
Thus, in the area (I) of FIG. 22, the particulates are oxidized and
removed without producing luminous flame for the short time and in
the area (II) of FIG. 22, the particulates are deposited to
laminate on the particulate filter. Therefore, the deposition of
the particulates on the particulate filter can be prevented if the
relationship between the amount of emitted particulates (M) and the
amount of particulates that can be oxidized and removed (G) is in
the area (I). AS a result, a pressure loss of the exhaust gas in
the particulate filter hardly changes and it is maintained at a
minimum pressure loss value that is nearly constant. Thus, the
decrease of the engine output can be maintained as low as possible.
However, this is not always realized, and the particulates may
deposit on the particulate filter if nothing is done.
In the present embodiment, to prevent the deposition of
particulates on the particulate filter, the above electronic
control unit 30 controls to change over the valve body 74 according
to a first flowchart shown in FIG. 24. The present flowchart is
repeated every a predetermined time. At step 101, the integrated
running distance (A) is calculated. Next, at step 102, it is
determined if the integrated running distance (A) is larger than a
predetermined running distance (As). When the result is negative,
the routine is stopped. However, when the result is positive, the
routine goes to step 103. At step 103, the integrated running
distance (A) is reset to 0 and at step 104, the valve body 74 is
changed over from one of the first position and the second position
to the other, that is, the upstream side and the downstream side of
the particulate filter are reversed.
FIG. 25 is an enlarged sectional view of the partition wall 54 of
the particulate filter. While the vehicle travels over the
predetermined running distance (As), the engine operation in the
area (II) of the FIG. 22 can be carried out. Thus, the particulates
collide with and are trapped by the exhaust gas upstream surface of
the partition wall 54 and the exhaust gas opposing surface in the
pores therein, i.e., one of the trapping surfaces of the partition
wall 54, and are oxidized and removed by active-oxygen released
from the active-oxygen releasing agent, but the particulates can
remain for the insufficient oxidization as shown by grids in FIG.
25(A). At this stage, the exhaust resistance of the particulate
filter does not have a bad influence on the traveling of the
vehicle. However, if the particulates deposit more, problems, in
which the engine output drops considerably, and the like, occur. By
the first flowchart, at this stage, the upstream side and the
downstream side of the particulate filter are reversed. Therefore,
no particulates deposits again on the residual particulates on one
of the trapping surfaces of the partition wall and thus the
residual particulates can be gradually oxidized and removed by
active-oxygen released from the one of the trapping surfaces.
Further, in particular, the residual particulates in the pores in
the partition wall are easily smashed into fine pieces by the
exhaust gas flow in the reverse direction as shown in FIG. 25(B),
and they mainly move through the pores toward the downstream
side.
Accordingly, many of the particulates smashed into fine pieces
diffuse in the pore in the partition wall, that is, the
particulates flow in the pore. Therefore, they contact directly the
active-oxygen releasing agent carried on the pores surface and thus
have many chances in which they are oxidized and removed. Thus, if
the active-oxygen releasing agent is also carried on the pores
surface in the partition wall, the residual particulates can be
very easily oxidized and removed. On the other trapping surface
that is now on the upstream side, as the flow of the exhaust gas is
reversed, i.e., the exhaust gas upstream surface of the partition
wall 54 and the exhaust gas opposing surface in the pores therein
to which the exhaust gas mainly impinges (of the oppose side of one
of the trapping surfaces), the particulates in the exhaust gas
adhere newly thereto and are oxidized and removed by active-oxygen
released from the active-oxygen releasing agent. In this
oxidization, a part of the active-oxygen released from the
active-oxygen releasing agent on the other trapping surface moves
to the downstream side with the exhaust gas, and it is made to
oxidize and remove the particulates that still remain on one of the
trapping surfaces despite of the reversed flow of the exhaust
gas.
That is, the residual particulates on one of the trapping surfaces
are exposed to not only active-oxygen released from this trapping
surface but also the remainder of the active-oxygen used for
oxidizing and removing the particulates on the other trapping
surface by reversing the flow of the exhaust gas. Therefore, even
if some particulates deposit laminate on one of the trapping
surfaces of the partition wall of the particulate filter when the
exhaust gas flow is reversed, active-oxygen arrives at the
deposited particulates and no particulates deposit again on the
deposited particulates due to the reversed flow of the exhaust gas
and thus the deposited particulates are gradually oxidized and
removed and they can be oxidized and removed sufficiently for some
period till the next reversal of the exhaust gas.
In the first flowchart, the valve body is changed over every the
predetermined running distance. However, the valve body may be
changed over every a predetermined period. Of course, the valve
body may not be periodically changed over in such a manner, but may
be irregularly changed over. In either case, it is preferable to
change over the valve body at least one time after the engine
starts and before the engine is stopped, such that the valve body
is changed over before the residual particulates transform into
carbonaceous matter that can hardly be oxidized. If the
particulates are oxidized and removed before the large amount of
particulates is deposit, problems, in which the large amount of
deposited particulates ignites and burns at once to melt the
particulate filter by the burned heat thereof, and the like, can be
prevented. Even if the large amount of particulates deposit on one
trapping surface of the partition wall of the particulate filter
for some reason when the valve body is changed over, the deposited
particulates is easily smashed into fine pieces by the reversed
flow of the exhaust gas. A part of the particulates that cannot be
oxidized and removed in the pores in the partition wall is
discharged from the particulate filter. However, therefore, it is
prevented that the exhaust resistance of the particulate filter
increases more to have a bad influence on the traveling of the
vehicle. Further, the other trapping surface of the partition wall
of the particulate filter can newly trap the particulates.
FIG. 26 shows a second flowchart for controlling to change over the
valve body 74. The present flowchart is repeated every a
predetermined time. At step 201, a pressure sensor detects an
exhaust pressure (P1) at one side of the particulate filter 70,
i.e., an exhaust pressure in the first flow passage 73a (refer to
FIG. 18). Next, at step 202, a pressure sensor detects an exhaust
pressure (P2) at the other side of the particulate filter 70, i.e.,
an exhaust pressure in the second flow passage 73b (refer to FIG.
18).
At step 203, it is determined if an absolute value of the
difference between the exhaust pressures detected at steps 201 and
202 is larger than a predetermined pressure difference (Ps). Here,
the absolute value of the difference pressure is used so that the
rise in the difference pressure can be detected if either of the
first flow passage 73a and the second flow passage 73b is the
exhaust gas upstream side. When the result at step 203 is negative,
the routine is stopped. However, when this result is positive, some
particulates remain on the particulate filter so that at step 204,
the valve body 74 is changed over and thus the upstream side and
downstream side of the particulate filter are reversed.
Accordingly, as the above mention, the residual particulates are
oxidized and removed from the particulate filter. Thus, utilizing
the difference pressure between the both sides of the particulate
filter, it is indirectly determined that some particulates remain
on the particulate filter and thus it can be certainly prevented
that the engine output drops much by the additional deposited
particulates. Of course, other than the difference pressure, for
example, observing the change of electric resistance on a
predetermined partition wall of the particulate filter, it may be
determined that some particulates deposit on the particulate filter
when the electric resistance becomes equal to or smaller than a
predetermined value by the deposition of the particulates. Besides,
utilizing the fact that a transmissivity or reflectivity of light
on a predetermined partition wall of the particulate filter drops
along with the deposition of the particulates thereon, it can be
determined that some particulates have deposited on the particulate
filter. If it is directly determined that the particulates remain
in such a manner and the valve body is changed over, it can be more
certainly prevented that the engine output drops. Strictly
speaking, the difference in pressure between the both sides of the
particulate filter changes in accordance with the pressure of the
exhaust gas discharged from the combustion chamber every engine
operating condition. Accordingly, in the determination of the
deposition of the particulates, it is preferable to specify the
engine operating condition.
Thus, the reverse of the upstream side and the downstream side of
the particulate filter is very effective to oxidize and remove the
residual and deposited particulates. Therefore, even if the valve
body is sometimes changed over without the determination of the
time, it can be favorably prevented that the engine output drops
much due to the large amount of deposited particulates.
By the way, in the structure of the valve body 74 of the present
embodiment, as mentioned above, a part of the exhaust gas bypasses
the particulate filter 70 during the changeover from one of the
first position and the second position to the other. Accordingly,
at this time, if the exhaust gas includes particulates, the
particulates are emitted into the atmosphere. To present this, as
in a third flowchart shown in FIG. 27, the valve body 74 may be
changed over when a fuel-cut is carried out. When a fuel-cut is
carried out, no combustion is carried out in the cylinder and thus
the exhaust gas includes no particulates. In the determination of
the execution of fuel-cut, a fuel-cut signal given to the fuel
injector may be utilized, the depression of brake pedal may be
detected while the vehicle is traveling, or the release of
accelerator pedal may be detected while the vehicle is
traveling.
According to the device of the present embodiment, the valve body
74 is changed over and the exhaust gas upstream side and the
exhaust gas downstream side of the particulate filter are reversed
so that the deposition of the particulates on the particulate
filter 70 can be favorably prevented. Further, according to the
structure of the present device, the circumferential portion 70c of
the particulate filter 70 is always in contact with the exhaust gas
in the third flow passage 73c, and thus it is heated by the exhaust
gas and the temperature of the particulate filter can be made high.
Therefore, as shown in the graph of FIG. 22, the amount of
particulates that can be oxidized and removed is maintained
relative large and thus the deposition of the particulates on the
particulate filter can be prevented more certainly. Further, if the
temperature of the particulate filter is made high, reducing
materials such as HC and CO that are slightly included in the
exhaust gas when the air-fuel ratio in the combustion is lean can
be favorably burned by using the oxidation catalyst carried on the
particulate filter. Therefore, the temperature of the particulate
filter can be raised higher.
The particulate filter 70 of the present embodiment has the large
opening portions 70a and 70b and the length between the opening
portions is relatively short. Therefore, a large amount of the
exhaust gas can pass through the particulate filter. Such a
particulate filter 70 is arranged in the exhaust pipe 71 such that
the opening portions are directed in the longitudinal direction of
the exhaust pipe. Therefore, the device can be made compact.
Accordingly, the device can be arranged at the immediate downstream
side of the exhaust manifold adjacently to the engine body and thus
heat of the exhaust can be utilized very effectively to heat the
particulate filter.
Further, in the deceleration and the like, when the temperature of
the exhaust gas is made low, the exhaust gas is made to bypass the
particulate filter by the opening position of the valve body 74,
and thus the exhaust gas can be prevented for passing through the
particulate filter. However, this low temperature exhaust gas flows
in contact with the circumferential portion of the particulate
filter and thus the temperature of the particulate filter drops in
the deceleration. After the deceleration, the high temperature
exhaust gas raises the temperature of the particulate filter soon.
Thus, in the deceleration, the amount of particulates that can be
oxidized and removed of the particulate filter drops. However, at
this time, the exhaust gas includes few particulates, and thus no
problem particularly occurs.
Besides, the whole particulate filter of the present embodiment is
surrounded by the flow of the exhaust gas, i.e., by a gas layer.
Thus, in comparison with a usual device for purifying the exhaust
gas that is adjacent to the atmosphere via a case, the device of
the embodiment can sufficiently suppress the heat release of the
particulate filter caused by wind during running. Therefore, the
temperature of the particulate filter can be easily maintained
high.
In the present embodiment, the particulate filter 70 is single and
has the oval sectional shape. However, this does not limit the
present invention. For example, as shown in FIG. 28, a plurality of
particulate filters 70' that have circular sectional shape or the
like may be arranged adjacently with one another in the
longitudinal direction of the exhaust pipe by the necessary number
of the particulate filters. In the single particulate filter, in
particular, the whole width of the partition wall in the center of
the height direction is made long and thus the rigidity of the
particulate filter is slightly low. On the other hand, by using a
plurality of the particulate filters, each particulate filter 70'
is made compact and thus has a high rigidity. Thus, the durability
of each particulate filter can be improved. Besides, the valve body
74 is arranged downstream of the exhaust pipe 71. This is an
advantage for the valve body, as a pivotable portion, to be away
from the high temperature engine body. However, of course, the
second small diameter portion 71b is positioned at the exhaust
upstream side and the valve body 74 may be upstream of the exhaust
pipe 71. Besides, in the present embodiment, the center line
passing through the first opening portion and the second opening
portion of the particulate filter 70 intersects with the center
line of the large diameter portion 71c of the exhaust pipe 71 at
right angles. However, this does not limit the present invention.
For example, the particulate filter 70 may not be positioned at the
center of the large diameter portion 71c and the opening portions
of the particulate filter 70 may be inclined with the center line
of the large diameter portion. Namely, if the center line of the
particulate filter may intersect with the center line of the large
diameter portion in plan view, the device having the above
mentioned effects can be constructed.
Further, when the air-fuel ratio of the exhaust gas is made rich,
i.e., when the oxygen concentration in the exhaust gas is
decreased, active-oxygen O is released at one time from the
active-oxygen releasing agent 61 to the outside. Therefore, the
deposited particulates become particulates that are easily oxidized
by the active-oxygen O released at one time and thus they can be
easily oxidized and removed.
On the other hand, when the air-fuel ratio is maintained lean, the
surface of platinum Pt is covered with oxygen, that is, oxygen
contamination is caused. When such oxygen contamination is caused,
the oxidization action to NO.sub.x of platinum Pt drops and thus
the absorbing efficiency of NO.sub.x drops. Therefore, the amount
of active-oxygen released from the active-oxygen releasing agent 61
decreases. However, when the air-fuel ratio is made rich, oxygen on
the surface of platinum Pt is consumed and thus the oxygen
contamination is cancelled. Accordingly, when the air-fuel ratio is
changed over from rich to lean again, the oxidization action to
NO.sub.x becomes strong and thus the absorbing efficiency rises.
Therefore, the amount of active-oxygen released from the
active-oxygen releasing agent 61 increases.
Thus, when the air-fuel ratio is maintained lean, if the air-fuel
ratio is changed over from lean to rich once in a while, the oxygen
contamination of platinum Pt is cancelled every this time and thus
the amount of released active-oxygen when the air-fuel ratio is
lean increases. Therefore, the oxidization action of the
particulates on the particulate filter 70 can be promoted.
Further, the result of the cancellation of the oxygen contamination
is that the reducing agent burns and thus the burned heat thereof
raises the temperature of the particulate filter. Therefore, the
amount of particulates that can be oxidized and removed of the
particulate filter increases and thus the residual and deposited
particulates are oxidized and removed more easily. If the air-fuel
ratio in the exhaust gas is made rich immediately after the
upstream side and the downstream side of the particulate filter is
reversed by the valve body 74, the other trapping surface on which
the particulates do not remain releases active-oxygen more easily
than the one trapping surface. Thus, the larger amount of released
active-oxygen can oxidize and removed the residual particulates on
the one trapping surface more certainly. Of course, the air-fuel
ratio of the exhaust gas may be sometimes made rich regardless the
changeover of the valve body 74. Therefore, the particulates hardly
remain or deposit on the particulate filter.
AS a method to make the air-fuel ratio rich, for example, the
above-mentioned low temperature combustion may be carried out. Of
course, when changing over from the normal combustion to the low
temperature combustion, or before this, the exhaust gas upstream
side and the exhaust gas downstream side of the particulate filter
may be reversed. Further, to make the air-fuel ratio of the exhaust
gas rich, the combustion air-fuel ratio may be merely made rich.
Further, in addition to the main fuel injection in the compression
stroke, the fuel injector may inject fuel into the cylinder in the
exhaust stroke or the expansion stroke (post-injection) or may
injected fuel into the cylinder in the intake stroke
(pre-injection). Of course, an interval between the post-injection
or the pre-injection and the main fuel injection may not be
provided. Further, fuel may be supplied to the exhaust system. As
above mentioned, the low temperature combustion is carried out in
the low engine load side and thus the low temperature combustion is
often carried out immediately after engine deceleration when a
fuel-cut is carried out. Therefore, in the third flowchart,
immediately after the valve body 74 is changed over, the low
temperature combustion is carried out and thus the air-fuel ratio
of the exhaust gas is frequently made rich.
By the way, when SO.sub.3 exists, calcium Ca in the exhaust gas
forms calcium sulfate CaSO.sub.4. Calcium sulfate CaSO.sub.4 is
hardly oxidized and removed and thus it remains on the particulate
filter as ash. To prevent blocking of the meshes of the particulate
filter caused by the remained calcium sulfate CaSO.sub.4, it is
preferable that an alkali metal or an alkali earth metal having an
ionization tendency stronger than that of calcium Ca, such as
potassium K is used as the active-oxygen releasing agent 61.
Therefore, SO.sub.3 diffused in the active-oxygen releasing agent
61 is combined with potassium K to form potassium sulfate K.sub.2
SO.sub.4 and thus calcium Ca is not combined with SO.sub.3 but
passes through the partition walls of the particulate filter.
Accordingly, the meshes of the particulate filter are not blocked
by the ash. Thus, it is desired to use, as the active-oxygen
releasing agent 61, an alkali metal or an alkali earth metal having
an ionization tendency stronger than calcium Ca, such as potassium
K, lithium Li, cesium Cs, rubidium Rb, barium Ba or strontium
Sr.
Even when only a noble metal such as platinum Pt is carried on the
particulate filter as the active-oxygen releasing agent,
active-oxygen can be released from NO.sub.2 or SO.sub.3 held on the
surface of platinum Pt. However, in this case, a curve that
represents the amount of particulates that can be oxidized and
removed (G) is slightly shifted toward the right compared with the
solid curve shown in FIG. 22. Further, ceria can be used as the
active-oxygen releasing agent. Ceria absorbs oxygen when the oxygen
concentration is high (Ce.sub.2 O.sub.3.fwdarw.2CeO.sub.2) and
releases active-oxygen when the oxygen concentration decreases
(2CeO.sub.2.fwdarw.Ce.sub.2 O.sub.3). Therefore, in order to
oxidize and remove the particulates, the air-fuel ratio of the
exhaust gas must be made rich at regular intervals or at irregular
intervals. Instead of the ceria, iron Fe or tin Sn can be used as
the active-oxygen releasing agent.
As the active-oxygen releasing agent, further, it is also allowable
to use an NO.sub.x absorbent for purifying NO.sub.x. In this case,
the air-fuel ratio of the exhaust gas must be made rich at least
temporarily to release and reduce the absorbed NO.sub.x and
SO.sub.x. It is preferable to make the air-fuel ratio rich after
the exhaust gas upstream side and the exhaust gas downstream side
of the particulate filter are reversed. The diesel engine of the
embodiments can change over the low temperature combustion and the
normal combustion. This does not limit the present invention. Of
course, the present invention can be also applied to a diesel
engine that carries out only normal combustion or a gasoline engine
that emits particulates.
In the present embodiment, the particulate filter itself carries
the active-oxygen releasing agent and active-oxygen released from
the active-oxygen releasing agent oxidizes and removes the
particulates. However, this does not limit the present invention.
For example, a particulates oxidization material such as
active-oxygen and NO.sub.2 that functions the same as active-oxygen
may be released from a particulate filter or a material carried
thereon, or may flow into a particulate filter from the outside
thereof. In case that the particulates oxidization material flows
into the particulate filter from the outside thereof, if the first
trapping surface and the second trapping surface of the partition
wall are alternately used to trap the particulates, on one trapping
surface that is now on the exhaust gas downstream side, no
particulates deposit newly on the residual particulates and the
residual particulates can be gradually oxidized and removed by the
particulates oxidization material flowing from the other trapping
surface and thus the residual particulates are perfectly removed
after some period. During this period, the other trapping surface
can trap the particulates and the trapped particulates are oxidized
and removed by the particulates oxidization material on the other
trapping surface. Thus, effects the same as the above-mentioned can
be obtained. Of course, in this case, if the temperature of the
particulate filter rises, the temperature of the particulates
themselves rises and thus the oxidizing and removing thereof can be
made easy. Therefore, it is advantage that the device is
constructed as the present embodiment.
According to the device for purifying the exhaust gas of the
present invention, the device comprises the particulate filter on
which the trapped particulates are oxidized and removed and the
reversing means for reversing the exhaust gas upstream side and the
exhaust gas downstream side of the particulate filter. The
particulate filter has a trapping wall for trapping the
particulates, the trapping wall has a first trapping surface and a
second trapping surface, and the reversing means reverses the
exhaust gas upstream side and the downstream side of the
particulate filter so that the first trapping surface and the
second trapping surface are used alternately to trap the
particulates. The particulate filter has the first opening portion
and the second opening portion through which the exhaust gas flows
in and out from the particulate filter, and is arranged in the
exhaust pipe upstream of the muffler. At least part of the
circumferential portion of the particulate filter between the first
opening portion and the second opening portion is in contact with
the flow of the exhaust gas in the exhaust pipe. Some particulates
can remain on the first trapping surface of the trapping wall of
the particulate filter due to the insufficient oxidization on the
particulate filter according to the engine operating condition.
However, the exhaust gas upstream side and the exhaust gas
downstream side of the particulate filter are reversed by the
reversing means so that no particulates deposit again on the
residual particulates on the first trapping surface and thus the
residual particulates can be oxidized and removed gradually. At the
same time, the second trapping surface of the trapping wall starts
to trap the particulates. Thus, if the first trapping surface and
the second trapping surface is used alternately to trap the
particulates, an amount of particulates trapped on each trapping
surface can be decreased to less than that in case where one
trapping surface always traps the particulates. This is an
advantage to oxidize and remove the particulates. Further, the
circumferential portion of the particulate filter is heated by the
exhaust gas that is in contact therewith and thus the temperature
of the particulate filter is maintained high so that the
particulates can come to be oxidized easily. Therefore, the
blocking of the meshes of the particulate filter can be certainly
prevented.
* * * * *